The speed and complexity of modern stock markets is partly driven by long-term investors who have adopted strategies pioneered by high-frequency traders, according to a top U.S. securities regulator.

Trading patterns analyzed by the Securities and Exchange Commission (SEC) found that money managers use sophisticated computer programs to fill their orders on a variety of private trading venues, including dark pools and over-the-counter markets. Their goals require "an unavoidable increase in the complexity of our markets, and in a very real sense [that] is also driving the need for more and faster technologies," said Gregg Berman, one of the SEC's top authorities on high-frequency trading.

High-speed trading is facing unprecedented scrutiny following the publication of Michael Lewis's book, "Flash Boys," which argues that the practice has helped rig the U.S. stock market. In a speech yesterday in New York, Berman didn't mention Lewis's book directly but said the current debate is "too narrowly focused and myopic."